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Low fuel pressure

I have less than 20 hrs of flight time on my recently purchased RV10. All of my time previous has been on high wing gravity feed systems. I have noticed on several occasions that my fuel pressure will start to fluctuate on long climbs and decent. Fuel flow and engine power seem to be unaffected. It happens regardless of which tank is selected. It always stabilizes at cruise. The pressure normally runs at 19 to 24. When it fluctuates, it can goes as low as 6 psi for brief periods. Turning on the electric pump immediately restores normal pressure so this leads me to believe it’s not a indication problem.

The transducer is located at the top of the firewall. I’ve read that two things can cause this. Air entering the system, or more common, fuel vapor collecting in the system. Some airplanes, particularly Diamonds claim this is normal and you should run the boost pump to clear the vapor until normal pressure returns.

Is this common in the RV10 and is it normal? If not, I will start checking for leaks, clean filters and screens and possibly have the mechanical pump overhauled.

Thanks
 
I think if you do a search on this you will find many posts on this subject.

I also had this issue start after 500 hours or so of flying. I replaced the mechanical pump to no avail. I determined it was vapor in the fuel lines and remedied this by eliminating two 90 degree fittings on the intake side of the fuel lines prior to the fuel tank selector. I fabricated a continuous aluminum line from the tank to the selector, which totally eliminated the issue. I know of several folks who have purchased a continuous flex line from TS Flightlines instead of using aluminum tubing. I also always leave the electric pump on in the climb.
Having said this, I would also check for leaks on the intake side of the line and contact the original builder and see if he experienced this issue. Wrapping each joint with a paper towel and leave for a couple of days is one way to check blue stains at the fittings and find leaks
 
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Robert----is this a new anomaly, or has it been consistant throughtout your ownership? How about the previous owner?
Tom
 
Very common on mine. I will get FP dips later in the climb, as the air gets thinner and it's capacity to cool is diminished. It is my opinion that the airflow dynamics in the cowl are very different in a lower speed climb attitude and the oil cooler exit air is blowing very near the fuel pump, creating vapor in the pump. Also possible it is the hot air coming out of the heater bypass valves hitting the fuel line right next to them. I also see spikes in oil temp during the climb, further indicating something is different in this attitude, as it drops as soon as I speed up in cruise. I rarely get spikes below 15 PSI and now just use the boost pump all the way up to cruise altitude. Boost pump fully eliminates the issue.

Vapor tends to only occur on the suction side of pumps, as the pressurized fuel naturally raises the boiling point. With the BP on, everything firewall forward is pressurized and will eliminate most vapor issues.

Larry
 
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Boost pump should not come off until Top of Climb. A cooling shroud on the fuel pump helps too.

Vic
 
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I think if you do a search on this you will find many posts on this subject.

I also had this issue start after 500 hours or so of flying. I replaced the mechanical pump to no avail. I determined it was vapor in the fuel lines and remedied this by eliminating two 90 degree fittings on the intake side of the fuel lines prior to the fuel tank selector. I fabricated a continuous aluminum line from the tank to the selector, which totally eliminated the issue. I know of several folks who have purchased a continuous flex line from TS Flightlines instead of using aluminum tubing. I also always leave the electric pump on in the climb.
Having said this, I would also check for leaks on the intake side of the line and contact the original builder and see if he experienced this issue. Wrapping each joint with a paper towel and leave for a couple of days is one way to check blue stains at the fittings and find leaks



Thanks for the reply Bill. I have heard several people mention this about 90 degree fittings. What I don’t understand is why a 90 degree fitting would cause vapor prior to the fuel selector. This is no where near the heat of the engine. What is causing the fuel to boil?
 
Robert----is this a new anomaly, or has it been consistant throughtout your ownership? How about the previous owner?
Tom

Tom, the previous owner is a interesting story. He basically couldn’t tell me much of anything about any of the systems or how they worked/performed. The plane was built by a professional builder, and I had a through prebuy done by Vic, so I know I have a solid plane. Ill give the previous owner a call and get his account of FP performance. That’s a good idea.
 
Thanks for the reply Bill. I have heard several people mention this about 90 degree fittings. What I don’t understand is why a 90 degree fitting would cause vapor prior to the fuel selector. This is no where near the heat of the engine. What is causing the fuel to boil?
Simple chemistry lesson- when the pressure of a liquid goes below its vapor pressure , it “boils’ or vaporizers
Flowing in tubing causes pressure drop. Tubing elbows also cause more pressure drop than straight tubing. 90 degrees fittings cause more than tubing turns. All that pressure drop puts the liquid below its vapor pressure
Cooling will reduce the vapor pressure so it can accept more pressure drop before it “boils” or vaporizes.

So smoother turns or add cooling will retard the pressure drop/vaporization and prevent the vapor lock you are seeing in the form of lower fuel pressure
 
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Quick update. I flew the plane yesterday and as Vic suggested, I left the boost pump on until TOC. Pressure stayed steady during climb. During cruise with the pump off, pressure also remained steady! That’s for everyone responses.
 
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